Re: [FFmpeg-devel] [PATCH 14/18] avcodec/hevcdec: Don't allocate redundant HEVCContexts

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

Michael Niedermayer:
> On Sat, Jul 02, 2022 at 08:32:06AM +0200, Andreas Rheinhardt wrote:
>> Michael Niedermayer:
>>> On Fri, Jul 01, 2022 at 12:29:45AM +0200, Andreas Rheinhardt wrote:
>>>> The HEVC decoder has both HEVCContext and HEVCLocalContext
>>>> structures. The latter is supposed to be the structure
>>>> containing the per-slicethread state.
>>>>
>>>> Yet up until now that is not how it is handled in practice:
>>>> Each HEVCLocalContext has a unique HEVCContext allocated for it
>>>> and each of these coincides except in exactly one field: The
>>>> corresponding HEVCLocalContext. This makes it possible to pass
>>>> the HEVCContext everywhere where logically a HEVCLocalContext
>>>> should be used. And up until recently, this is how it has been done.
>>>>
>>>> Yet the preceding patches changed this, making it possible
>>>> to avoid allocating redundant HEVCContexts.
>>>>
>>>> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
>>>> ---
>>>>  libavcodec/hevcdec.c | 40 ++++++++++++++++------------------------
>>>>  libavcodec/hevcdec.h |  2 --
>>>>  2 files changed, 16 insertions(+), 26 deletions(-)
>>>>
>>>> diff --git a/libavcodec/hevcdec.c b/libavcodec/hevcdec.c
>>>> index 9d1241f293..048fcc76b4 100644
>>>> --- a/libavcodec/hevcdec.c
>>>> +++ b/libavcodec/hevcdec.c
>>>> @@ -2548,13 +2548,12 @@ static int hls_decode_entry_wpp(AVCodecContext *avctxt, void *hevc_lclist,
>>>>  {
>>>>      HEVCLocalContext *lc = ((HEVCLocalContext**)hevc_lclist)[self_id];
>>>>      const HEVCContext *const s = lc->parent;
>>>> -    HEVCContext *s1  = avctxt->priv_data;
>>>> -    int ctb_size    = 1<< s1->ps.sps->log2_ctb_size;
>>>> +    int ctb_size    = 1 << s->ps.sps->log2_ctb_size;
>>>>      int more_data   = 1;
>>>>      int ctb_row = job;
>>>> -    int ctb_addr_rs = s1->sh.slice_ctb_addr_rs + ctb_row * ((s1->ps.sps->width + ctb_size - 1) >> s1->ps.sps->log2_ctb_size);
>>>> -    int ctb_addr_ts = s1->ps.pps->ctb_addr_rs_to_ts[ctb_addr_rs];
>>>> -    int thread = ctb_row % s1->threads_number;
>>>> +    int ctb_addr_rs = s->sh.slice_ctb_addr_rs + ctb_row * ((s->ps.sps->width + ctb_size - 1) >> s->ps.sps->log2_ctb_size);
>>>> +    int ctb_addr_ts = s->ps.pps->ctb_addr_rs_to_ts[ctb_addr_rs];
>>>> +    int thread = ctb_row % s->threads_number;
>>>>      int ret;
>>>>  
>>>>      if(ctb_row) {
>>>> @@ -2572,7 +2571,7 @@ static int hls_decode_entry_wpp(AVCodecContext *avctxt, void *hevc_lclist,
>>>>  
>>>>          ff_thread_await_progress2(s->avctx, ctb_row, thread, SHIFT_CTB_WPP);
>>>>  
>>>> -        if (atomic_load(&s1->wpp_err)) {
>>>> +        if (atomic_load(&s->wpp_err)) {
>>>>              ff_thread_report_progress2(s->avctx, ctb_row , thread, SHIFT_CTB_WPP);
>>>
>>> the consts in "const HEVCContext *const " make clang version 6.0.0-1ubuntu2 unhappy
>>> (this was building shared libs)
>>>
>>>
>>> CC	libavcodec/hevcdec.o
>>> src/libavcodec/hevcdec.c:2574:13: error: address argument to atomic operation must be a pointer to non-const _Atomic type ('const atomic_int *' (aka 'const _Atomic(int) *') invalid)
>>>         if (atomic_load(&s->wpp_err)) {
>>>             ^           ~~~~~~~~~~~
>>> /usr/lib/llvm-6.0/lib/clang/6.0.0/include/stdatomic.h:134:29: note: expanded from macro 'atomic_load'
>>> #define atomic_load(object) __c11_atomic_load(object, __ATOMIC_SEQ_CST)
>>>                             ^                 ~~~~~~
>>> 1 error generated.
>>> src/ffbuild/common.mak:81: recipe for target 'libavcodec/hevcdec.o' failed
>>> make: *** [libavcodec/hevcdec.o] Error 1
>>>
>>> thx
>>>
>>
>> Thanks for testing this. atomic_load is indeed declared without const in
>> 7.17.7.2:
>>
>> C atomic_load(volatile A *object);
>>
>> Upon reflection this makes sense, because if atomics are implemented via
>> mutexes, even a read may involve a preceding write. So I'll cast const
>> away here, too, and add a comment. (It works when casting const away,
>> doesn't it?)
> 
> This doesnt feel "right". These pointers should not be coming from a const
> if they are written to
> 

The HEVCContext is not const because the underlying object is const; the
HEVCContext is const when accessed from any part of the code that may be
run from slice threads, because if a slice thread modifies it, you have
a data race in case any of the other slice threads reads this field or
modifies it itself. But this is by definition not true for atomic
operations, so casting const away for them is fine.

> The compiler accepts it with an explicit cast though. With an implicit cast
> it produces a warning
> 
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